1. Field of the Invention
This invention relates to a high voltage generating apparatus for a television receiver, or more in particular to a high voltage generating apparatus with an improved focus voltage supply circuit.
2. Description of the Prior Art
In a picture tube which is the so-called bipotential focus lens type used for the television receiver or like, a D.C. focus voltage in proportion to the D.C. anode voltage is required. The optimum value of focus voltage for the color picture tube of the conventional television receivers is 18% to 21% of the anode voltage depending on the picture tube involved. Therefore, the high voltage generating apparatus for such a receiver must be provided with a focus voltage generator circuit capable of generating a voltage in a certain variable range, in addition to an anode voltage generation circuit.
Various focus regulator circuits are well known. With expensive and bulky high voltage resistors or regulating transformer, they are high in cost and require a large space, and because of their high working voltages, they are disadvantageous to reliability and operability.
A fundamental circuit configuration of a conventional well known high voltage generating apparatus is shown in FIG. 1. Reference numeral 1 designates a horizontal output tube, numeral 2 a damper tube, numeral 3 a flyback transformer, numeral 4 a high voltage rectifier tube for anode D.C. voltage, numeral 5 a focus diode, numeral 6 a focus capacitor, numeral 7 a focus-regulating transformer, numeral 8 a movable core of the focus regulating transformer 7, numeral 9 a focus breeder resistor, and numeral 10 an anode-to-earth capacitance of the picture tube. A sectional view of FIG. 2 shows the construction of the focus regulating transformer 7. In this drawing, numeral 11 designates a coil bobbin, numeral 12 coils and numeral 8 a regulating movable core.
The flyback transformer 3 is an auto-transformer having a tap A at a point where the flyback pulse voltage during the blanking period is substantially equal to the required focus D.C. voltage. The flyback pulse is rectified by the focusing diode 5 and the focusing capacitor 6 thereby to produce a focus D.C. voltage E.sub.F. The other end D of the focusing capacitor 6 is impressed with a variable flyback pulse voltage obtained from the focus regulating transformer 7, thus regulating the focus voltage E.sub.F. The focus regulating transformer 7, as shown in FIG. 2, comprises three coils L.sub.1, L.sub.2 and L.sub.3. The coils L.sub.1 and L.sub.3 are connected in opposite phase, and across them is impressed a flyback pulse generated between the tap B and the starting point C of the winding of the flyback transformer 3. Further, the coil L.sub.2, which is provided in the same magnetic circuit, has an end connected to a junction point of the coils L.sub.1 and L.sub.3, and the other end D connected to the low-voltage side of the focusing capacitor 6. When the regulating core 8 is moved in the direction shown by the arrow, the coupling between the coils L.sub.1, L.sub.2 and L.sub. 3 changes and therefore the flyback pulse voltage generated in the coil L.sub.2 also changes, thus making it possible to regulate the magnitude of the flyback pulse voltage at point D. By the way, the focus power supply has a high breakdown voltage and is grounded through the focus breeder resistor 9. The purpose of this is to improve the tracking characteristics and response speed of the focus voltage E.sub.F against anode voltage E.sub.HV. The internal impedance of the focus electrode is almost infinitely large so that, in the absence of the focus breeder resistor 9, even if the anode voltage E.sub.HV is reduced, charges stored in the focus capacitor 6 have no discharge path, thus maintaining the focus voltage E.sub.F at high level.
The focus regulating transformer 7 which are coils used in the above-mentioned conventional method handles a comparatively high pulse voltage for its capacity of approximately 1.5 KV. To maintain high reliability, it is both bulky and expensive. Further, the voltage regulating system has such a construction as shown in FIG. 2 that the core is moved extensively by rotation in combination with a bobbin having a threaded groove, resulting in the disadvantage of the complicated regulating operation. Furthermore, a large-sized high-cost resistor with a high breakdown voltage is required as the focus breeder resistor 9, thus leading to the shortcomings in respect of cost, reliability, safety and mounting space.
FIG. 3 shows a focus regulator circuit often used for a transistorized high voltage generating apparatus. In this drawing, numeral 13 designates a horizontal output transistor, numeral 14 a damper diode, numeral 15 a resonate capacitor, numeral 16 a deflection yoke, numeral 17 an S-shaped correction capacitor, numeral 18 a flyback transformer, numeral 19 a primary winding, 20 a high voltage winding, numeral 21 a high voltage diode, 22 and 24 high voltage resistors, and numeral 23 a variable resistor. As will be seen from FIG. 3, the focus D.C. voltage E.sub.F in the form of the desired variable voltage proportional to the anode D.C. voltage E.sub.HV is obtained by dividing the anode D.C. voltage E.sub.HV directly by the high voltage resistors 22 and 24 and the variable resistor 23.
This circuit, though capable of producing a required voltage with comparative ease, has many disadvantages. In a color picture tube which requires a high anode voltage E.sub.HV of 20 to 25 KV, for instance, the voltage-dividing resistors 22, 23 and 24 and their mounting structure must be sufficiently insulated to stand the high voltage. This circuit has the disadvantages that it is in need of a high insulating ability, high cost a bulkiness on the one hand and a comparatively high power consumption by the resistors on the other.
Another conventional circuit partially improved over the circuit of FIG. 3 is shown in FIG. 4. In the drawing under consideration, the secondary winding of the flyback transformer 18 is divided into two parts 25 and 26. The first part 25 of the secondary winding is connected in series with the diode 27 therefor and the focus capacitor 29 with one end thereof grounded. A DC voltage generated by the focus capacitor 29 is divided by the high voltage resistors 30 and 32 and the variable resistor 31 thereby to produce a variable focus voltage. Also, an end of the first part 26 of the secondary winding is connected to the cathode of the diode 27 for the first part of the secondary winding, and thereby the focus D.C. voltage is used as part of the anode D.C. voltage. A separate diode 28 is used for rectification of the voltage across the first part of the secondary winding. This method reduces the voltage applied to the resistors by approximately one-fourth of the voltage in the circuit of FIG. 3, but still has the same shortcomings as the circuit of FIG. 3.